This invention relates to an antenna device and, in particular, to an antenna device for being used by mounting it on a roof of a mobile unit (a car body) such as an automobile and to an antenna for use in the same. Such an antenna device is, for example, an antenna device for use in a digital radio receiver for receiving a radio wave from an artificial satellite (that may be also called a “satellite wave” hereinafter) or a radio wave on a ground station (that may be also called a “terrestrial wave” hereinafter) to listen to a digital radio broadcasting.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave so as to listen to the digital radio broadcasting, has been developed and is put to practical use in the United States of America. Specifically, two broadcasting stations called XM and Sirius provide radio programs on 250 or more channels in total. The digital radio receiver is generally mounted on a mobile unit such as an automobile and is adapted to receive a radio wave having a frequency of about 2.3 gigahertz (GHz) as a received wave to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) A thereof is equal to about 128.3 mm. It is noted here that the terrestrial wave is a radio wave obtained by receiving the satellite wave at a ground station, slightly shifting the frequency of the satellite wave, and retransmitting the linear polarized wave. Thus, the terrestrial wave is the linear polarized wave exhibiting linear polarization while the satellite wave is a circular polarized wave exhibiting circular polarization.
An XM satellite radio antenna apparatus normally serves to receive circular polarized radio waves from two stationary satellites and, in an insensitive zone of the circular polarized waves, receives a radio wave by using a terrestrial linear polarization portion of the radio antenna apparatus. On the other hand, a Sirius satellite radio antenna apparatus normally serves to receive circular polarized radio waves from three orbiting satellites (synchronous type) and, in the insensitive zone, receives a radio wave by a terrestrial linear polarization portion of the radio antenna apparatus.
As described above, the radio wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting. Therefore, an antenna for receiving the radio wave is often located outside as known in the art. If the digital radio receiver is mounted in the mobile unit such as the automobile, the antenna device is generally attached to a roof of the mobile unit (car body).
The antenna device comprises an antenna and an antenna case for covering the antenna. The antenna case comprises a dome-like top cover and a bottom plate (a base). The antenna comprises an antenna element, a circuit board, and a shield case. For example, the antenna element may comprise a patch antenna for receiving the satellite antenna. The antenna element and the circuit board are directly connected to each other through a double-faced tape. The circuit board has a rear surface on which a circuit (hereinafter will be called a signal processing circuit) for performing various kinds of signal processing, such as signal amplification, upon a signal received by the antenna element is formed (mounted). The shield case serves to shield the signal processing circuit.
In such an antenna device mounted on the mobile unit, a mechanism (unit fixing member) for fixing the antenna device to the roof of the mobile unit (car body) must be formed by a component inside the antenna device. In addition, inasmuch as the antenna must be disposed inside the antenna case, another component (antenna fixing member) for fixing the antenna to the bottom plate is required also.
U.S. Pat. No. 7,365,694, which will be called Patent Document 1 hereinafter, discloses an antenna unit provided with a unit fixing member disposed between an antenna and a bottom plate. The unit fixing member is adapted to provisionally fix the antenna unit onto a mobile unit and to fix the antenna onto the bottom plate. In Patent Document 1, a shield case has a plurality of tabs for mounting the antenna to the bottom plate.
In the antenna device disclosed in Patent Document 1, the unit fixing member which is complicated in structure and is thick is disposed between the antenna and the bottom plate. Accordingly, on covering the antenna with a top caver, the height of the antenna device becomes high as a whole.
In addition, inasmuch as the antenna device is mounted on the roof of the mobile unit (car body), vibrations and impact of the mobile unit are delivered to the antenna accommodated in the antenna device (antenna case). In addition, the antenna is also subjected to acceleration and deceleration of the mobile unit in the direction of travel. That is, the antenna device is used in a hostile environment. In the manner which is described above, in a conventional antenna, the antenna element and the circuit board are directly connected to each other. In other words, the antenna element is only mounted (fixed) on the circuit board and is never supported to the bottom plate (base). Accordingly, in the conventional antenna, stress subjected to the vibrations and impact of the mobile unit or the acceleration and deceleration is exerted to the antenna element in itself, Specifically, in a case where soft material such as Teflon (registered trademark) is uses as a material of a high-frequency board making up the antenna element, the antenna element becomes deformed when the above-mentioned stress is exerted thereto. This is because the antenna element itself is never supported on the base.